1. Field of the Invention
The present invention relates to a radio communication terminal, a base station, a handoff control method and a base station control method that are employed in a radio communication system compatible with a broadcast service and/or a multicast service, for example.
2. Description of the Related Art
1xEV-DO (1x Evolution Data Only) communication system has been commercialized, and one of the system functions is a broadcast multicast service (BCMCS).
In the broadcast service, a radio communication network is used, and a base station continuously transmits data in a connectionless form to all radio communication terminals within a service area (for example, a cell or a sector) of the base station. That is, the broadcast service is one form of services of “broadcasting.”
The multicast service is similar to the broadcast service. The multicast service is one form of a service in which broadcasting is performed so that only specific users among the radio communication terminals within the service area who have signed up for specified service information (for example, a fee-based service) can receive the data.
FIG. 1 is a diagram showing a broadcast data and user data transmission schedule in the 1xEV-DO communication system. FIG. 2 is a diagram showing a broadcast data transmission schedule for each base station in the 1xEV-DO communication system.
In the broadcast service provided by the 1xEV-DO communication system, unlike a unicast data communication, the base station distributes the same data for all the users requesting for the data (FIG. 1).
Depending on a system, the base station synchronizes or does not synchronize the broadcast data transmission schedule for a unit of multiple base stations or for a unit of multiple sectors according to the base station.
For example, in a base station A shown in FIG. 2, broadcast schedules of the broadcast data in sectors α, β and γ are the same. In a base station B, broadcast scheduling is made for each sector.
In case where the broadcast data to be broadcasted has an excessively large data size, such data is expected to be divided into smaller packets and broadcasted in the smaller packets as shown in FIG. 3. The divided packets are expected to be assigned with a sequence number in an upper layer to be managed so that the divided packets can be complemented in decoding.
In a radio communication system, as a technique for connecting the communication while a radio communication terminal moves across service areas (for example, cells or sectors) each covered by a base station, processing is performed to switch between base stations in response to a communication quality between the radio communication terminal and the base station. This processing is called handoff processing. The radio communication terminal measures both an intensity of a signal transmitted from the base station under connection and an intensity of a signal transmitted from each of the peripheral base stations in order to perform the processing for maintaining more appropriate radio wave conditions.
For example, in a case of a CDMA (Code Division Multiple Access) radio communication system of IS95 system, the radio communication terminal measures the signal intensity (communication quality) of a signal coming from the base station currently under connection with the radio communication terminal and the signal intensity of a signal coming from each of the peripheral base stations. Then, while the radio communication terminal is under communication, the radio communication terminal transmits the measured signal intensity to the base station. Then, the base station determines whether the handoff is to be performed based on the transmitted signal intensity. On the other hand, when the radio communication terminal is in a standby mode, the radio communication terminal itself determines whether the handoff is to be performed based on the measured signal intensity.
Generally, in the 1xEV-DO communication system, while the handoff operation is typically performed by the terminal when the terminal is in the standby mode, the terminal does not notify the base station of the handoff before and after the handoff is performed.
During the communication, the base station once assigns a handoff destination, and after that, the terminal performs the handoff. In this case, the terminal notifies the base station of the handoff before and after the handoff is performed (FIG. 10: Sequence 1: Handoff under communication).
However, “under reception of broadcast data” does not necessarily mean “under communication”. Therefore, a terminal that is not involved in a usual unicast data communication, even though the terminal is under reception of the broadcast data, does not necessarily notify the base station when the handoff is performed.
In a case where the handoff is made between areas where the schedule of the broadcast data is synchronized under the reception of the broadcast data, the handoff operation in the 1xEV-DO communication system requires approximately a period of 150 slots at the maximum when the handoff is made. Thus, the broadcast data cannot be received while the handoff is performed.
In a case where the schedule of the broadcast data is not synchronized between the areas involved in the handoff, and the base station of the handoff destination has not yet delivered the broadcast data after the handoff is performed, delivery is made from the data which is already received (from the beginning) as shown in FIG. 4.